Thermo-mechanical Fatigue of Discontinuously Reinforced Light Metals

Combining a high strength ceramic phase with a matrix of ductile light metal to form metal matrix composites (MMC) which offer attractive potential for use in light weight engines because of improved strength, stiffness, dimensional stability, fatigue resistance, and elevated-temperature property retention compared with unreinforced alloys. The use of ceramic particles as reinforcement (PRM) has the key advantage that they can be fabricated to final products using conventional processing routes such as casting, forging or extrusion. In components, which are mechanically loaded while the temperature is changing during service, such as pistons, cylinder liners or brake components, the understanding of the thermo-mechanical fatigue behaviour is essential to design the composite and the component to suit specific applications with high reliability. . The project "Thermo-mechanical fatigue of discontinuously reinforced light metals" has the aim to generate basic knowledge about the behaviour of particulate reinforced light metals under changing thermal conditions. Useful life-time data of material behaviour at service temperature are necessary for the design of such components and there are still significant deficiencies. This research project consists of broad experimental thermo-mechanical fatigue tests on aluminium alloys, both unreinforced and various reinforced matrices in temper conditions of practical use. The expected results in life-time are dependent on thermo-mechanical loading history. Material degradation and damage mechanisms will be investigated in correlation with the different material conditions. Understanding of material failure under thermo-mechanical loading is of significant importance for the prediction of fatigue lifetime, because internal stresses are changing according to the mismatch in thermal expansion of the composites` components. The experimental results will be interpreted by correlating systematically the microstructural responses to the loading conditions. Thermo-mechanical in-situ experiments by the use of a scanning electron microscope and a miniaturized testing device enables to investigate the material failure "in statu nascendi" and the exact correlation of various failure mechanisms to the thermal and mechanical loading stage.

In the case of the investigated aluminum alloys a reinforcement with ceramic particles causes a deterioration of the creep resistance not only in thermocycling, but also in isothermal creep, which can be explained by the effect of the internal stresses. Short Fibres as reinforcing component improve the creep resistance of the aluminium alloy, whereas the influence of heat treatment condition on creep properties vanishes. The behaviour of discontinously reinforced Metall Matrix Composites is investigated by thermocycling creep tests with three different materials. Two particle reinforced aluminum matrix composites, a wrought alloy and a cast alloy, were produced by a molten metal mixing process ("stir-casting") and hot extruded by CoC on Lightmetals Ranshofen. A short fibre reinforced aluminum piston alloy was produced by direct squeeze casting at TU Clausthal. The thermomechanical fatigue tests were carried out with the Gleeble 1500 apparatus, where the materials are loaded in constant tension and thermally cycled between 50 and 300C up to failure. Heating and cooling rates of 12,5 K/s can be obtained by direct resistance heating and the cooling with pressurized air, (total cycle time of 46 seconds). The most important results of the thermocycling creep tests are the minimun creep rate, which is reached during secondary creep, and the number of thermal cycles to failure. The particle reinforcement causes an obvious deterioration of the creep resistance, the life time is reduced significantly compared to unreinforced samples, which are tested under same conditions. The particle reinforced cast alloy exhibits a higher life cycle than the unreinforced alloy only for small loads due to a significant increase in ductility. Already realised applications of particle reinforced MMC are brake rotors or brake drums for cars and the brake rotors for the German high speed train ICE-2. The reinforcement with ceramic short fibres causes an increase of life time for all testing conditions. Compared to the unreinforced samples, the influence of heat treatment conditon becomes negligible because of the short fibre reinforcement. The results of the thermal cycling creep tests can be generalized by modified material laws for isothermal creep. Inner tensions caused by the difference in the coefficient of thermal expansion have a positiv influence on the fatigue behaviour of the material. It was possible to correlate the progression of failure with different fibre breaking mechanisms by characterisation of the fibre length, metallographic investigations and experiments with the insitu tensile/heating stage in the SEM. Pistons of some truck Diesel engines are already selectivly reinforced by short fibre alumina fibres.